WASHINGTON -- The Atlantic Meridional Overturning Circulation -- also known as the conveyor belt -- was featured in the documentary "An Inconvenient Truth" and the disaster flick "The Day After Tomorrow" as a changeable force that could wreak havoc on the climate in Europe and North America if it slowed down.

Now scientists are tracking the massive flows of shallow warm and deep cold ocean water that make up the current. They are taking detailed measurements in a line stretching across the Atlantic from the Bahamas to Africa, researchers wrote Thursday in the journal Science.

The current is called the conveyor belt because it forms a giant loop from the Gulf of Mexico to Iceland and back. Warm water -- the Gulf Stream -- flows north near the ocean surface along the U.S. East Coast, then veers northeast before breaking into several streams.

Some of these continue north past Iceland while the rest return south.

On its northward journey, the warm water cools and becomes more dense, sinking as it gets to extreme northern latitudes and returning southward at depths down to 3 miles. It also becomes less salty, fueled by fresh water runoff from rain and melting glaciers, including the Greenland Ice Sheet.

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A year of observation indicates the circulation system may vary widely over 12 months, the scientists reported.

There is not yet enough data to tell whether global warming is having an impact, said study co-author Stuart Cunningham of the National Oceanography Centre in Britain.

"I think it's too soon to tell," he told Reuters. "Basically, the previous observations have been snapshot estimates so there is quite a large amount of uncertainty associated with interpreting these estimates."

But the detailed findings agree with previous estimates, which indicates no "dramatic changes" so far.

Cunningham said climate models suggest that changes caused by humans to the current will be relatively steady, slowing down the conveyor belt during the next 50 years.

However, he said, "A lot of paleoclimate evidence suggests that transitions can be rather large and abrupt, maybe 50 percent changes over a few years, and if that happens we'd see it immediately."

To track the current, researchers placed a series of anchored instruments at latitude 26.5 degrees north, some clustered in the western Atlantic near the Bahamas, others off the north African coast.

These instruments measure changes in pressure, temperature and salinity throughout the water column from the surface to the sea bottom. The published study offers measurements from March 2004 through March 2005, but the instruments will continue observing at least through 2014.